Young adults who vape are more than four times as likely to start smoking cigarettes within 18 months as their peers who don’t vape, researchers report.

The findings demonstrate that e-cigarettes are serving as a gateway to traditional smoking, contrary to their purported value as a smoking cessation tool.

“This supports policy and educational interventions designed to decrease the use of e-cigarettes among nonsmokers.”

The study, which appears in the American Journal of Medicine, is the first nationally representative survey that followed for more than a year people 18 to 30 years old who were initially nonsmokers.

“Early evidence on the potential value of e-cigarettes for cessation or reduction of cigarette smoking has been mixed,” says lead author Brian A. Primack, director of the Center for Research on Media, Technology, and Health at the University of Pittsburgh.

“Our study finds that in nonsmokers, e-cigarettes make people more likely to start smoking. This supports policy and educational interventions designed to decrease the use of e-cigarettes among nonsmokers.”

The team analyzed a survey of US adults who were randomly selected in March 2013 to complete a questionnaire about their tobacco use. Eighteen months later, in October 2014, 915 participants who says they had never smoked cigarettes completed a follow-up survey.

The team then applied “weights” to the survey results by over- and under-emphasizing the answers of the survey participants in order to arrive at findings that would be more representative of the true make-up of the US population. For example, only 14.2 percent of those surveyed were Hispanic, so the team over-emphasized their answers so that the weighted sample and final results were 19.7 percent Hispanic.

The final, weighted survey results showed that 11.2 percent of participants—none of whom had ever smoked when they completed the initial questionnaire—had started smoking tobacco cigarettes. Of participants who says they vaped e-cigarettes in the first questionnaire, 47.7 percent had started smoking cigarettes 18 months later, compared to 10.2 percent of those who did not use e-cigarettes.

Without the survey weights to make the sample representative of the US population, 37.5 percent of e-cigarette users had started smoking cigarettes 18 months later, compared to 9 percent of those who didn’t use e-cigarettes.

More research will be needed to determine why e-cigarettes increase the risk of someone transitioning to tobacco cigarettes, but Primack notes that several factors are likely at play, including that using e-cigarettes mimics the behavior of smoking traditional cigarettes, the sweet vape is a gentle introduction to smoking harsher tobacco, and the build-up of nicotine addiction could lead e-cigarette users to seek out more nicotine-packed tobacco cigarettes.

“Young adulthood is an important time when people establish whether they use tobacco or not,” says Primack, also a professor of medicine, pediatrics, and clinical and translational science at Pitt’s School of Medicine. “Our findings suggest that clinicians who treat e-cigarette users should counsel them both about their potential for harm and about the high risk of transitioning to tobacco cigarettes among initial nonsmokers.”

Additional authors of the study are from the University of Pittsburgh; the VA Pittsburgh Healthcare System; and Dartmouth College. The National Cancer Institute funded the research.